1. Field of the Invention
This invention relates to a magnetic recording medium comprising a magnetic layer suitable for high density magnetic recording, and to a magnetic recording apparatus using this medium.
2. Description of the Related Art
Magnetic disk apparatuses currently in use employ the in-plane magnetic recording method. In this method, there is a technical problem in forming an in-plane magnetic domain parallel to a substrate on an in-plane magnetic recording medium which is easily magnetized in a direction parallel to the disk substrate surface. In in-plane recording, as the magnetizations are adjacent to each other in mutually opposite directions, the thickness of the recording layer must decrease as the coercivity of the layer is increased so as to extend the linear recording density. Due to thermal fluctuations when the thickness of the recording layer decreases, the intensity of recorded magnetization decreases and in extreme cases, the recorded information may be lost. Also, in the in-plane recording method, if Co alloy is used for the recording layer as in the prior art, it is difficult to achieve an areal recording density of not less than 20 Gb/in2.
In the perpendicular magnetic recording method, magnetizations are perpendicular to the surface of a film medium, so the magnetic recording principle and the mechanism whereby noise arises from the medium differs from the case of prior art in-plane magnetic recording media. Due to the fact that magnetizations are in complementary directions, this method is suitable for high density magnetic recording. It is therefore becoming more common and various structures have been proposed for media suitable for perpendicular magnetic recording. A method is presently been studied where a non-magnetic base material is provided between a perpendicular magnetized layer of Co alloy and a substrate to improve the perpendicular orientation characteristics of the perpendicular magnetized layer. For example, in JP-A No. S58-77025 and No. S58-141435, a method is disclosed for forming a Ti film as the base layer of a Coxe2x80x94Cr magnetic layer, in JP-A No. S60-214417, a method is disclosed using Ge or Si as the base layer, and in JP-A No. S60-064413, oxide base layer materials such as CoO and NiO are used.
These single-layered perpendicular magnetic recording media comprising a single perpendicular magnetic layer employ a thin film ring head for recording.
To improve the recording efficiency of perpendicular magnetic recording, it is effective to combine a single pole type of recording head with a perpendicular magnetic recording medium having two magnetic layers. A medium wherein a soft magnetic layer of permalloy or Co alloy is provided between the substrate and the perpendicular magnetic layer has been studied as an example of a bi-layered perpendicular magnetic recording medium. However, in bi-layered perpendicular magnetic recording media, the intensity of perpendicular magnetic anisotropy of the recording magnetic layer was inadequate compared to single-layered perpendicular magnetic recording media.
If a perpendicular magnetic recording medium is to achieve high density recording of 20 Gb/in2 or more, the linear recording density resolution must be high, the noise due to the medium must be low, and recording must be performed efficiently by a thin film head. For this purpose, the perpendicular magnetic layer must have a fine magnetic crystal grain, the perpendicular magnetic anisotropy must be increased, and the recording magnetic field of the magnetic head must effectively penetrate inside the medium.
It is therefore an object of this invention to provide a perpendicular magnetic recording medium which has high resolution so as to achieve a high recording density of 30 Gb/in2, has low noise characteristics, and permits a high density magnetic recording apparatus to be easily constructed.
Perpendicular magnetic recording media which offer high recording efficiency by a magnetic head are bi-layered perpendicular magnetic recording media. According to this invention, to achieve the aforesaid object, a super-thin MgO film is introduced between a soft magnetic backlayer formed on a substrate and a Co alloy perpendicular magnetic layer having a hexagonal close-packed structure. To ensure that the object of this invention is achieved, a super-thin non-magnetic film of a special material is formed in the lower part and/or upper part of the MgO film.
The soft magnetic backlayer used in the bi-layered perpendicular magnetic recording medium is generally a polycrystalline film having a permalloy of Ni or the like as its main component, a polycrystalline film like Sendust having Fe as its main component, or a Co alloy such as Coxe2x80x94Nbxe2x80x94Zr. When the perpendicular magnetic layer of Co alloy is formed directly on this soft magnetic backlayer, in the initial growth state of the thin film, it contains an initial growth layer which is undesirable for a perpendicular magnetic layer wherein the crystal growth is random. As a result, there is a decrease of perpendicular magnetic anisotropy, and the magnetic separation between the magnetic crystal grains forming the perpendicular magnetic layer is insufficient, which leads to a decrease of coercivity or increase of noise.
The inventors found that the introduction of the super-thin film of MgO between the soft magnetic back layer and the perpendicular magnetic layer comprising Co alloy was effective in dealing with this problem. When the MgO film is formed on the soft magnetic backlayer having an amorphous structure, MgO microcrystalline grains are formed wherein the (100) plane is substantially parallel to the substrate, and as a result, a polycrystalline MgO oriented film grows wherein the (100) plane is essentially disposed parallel to the substrate. The (100) plane of MgO is energetically most stable crystallographic plane, and thus the (100) oriented MgO crystal grains tend to be formed when deposited on a flat surface. When a substrate has surface undulations, the (100) MgO plane has a slight misorientation, but substantially the (100) plane is almost parallel to the substrate surface. When the perpendicular magnetic layer of Co alloy is formed on this oriented film, magnetic crystal grains grow having a hexagonal close-packed structure with an easily magnetization [0001] axis perpendicular to the substrate, and perpendicular magnetic anisotropy therefore increases.
The thickness of the MgO film required to produce this effect is 1 nm or greater. If the thickness of the MgO film is made too large, the distance between the soft magnetic backlayer and perpendicular magnetic layer increases, so recording efficiency when recording is performed by a magnetic head decreases. The linear recording density required to achieve an areal recording density of 30 Gb/in2 or more is at least 300 kFCI, and to increase the efficiency of the recording head at this high linear recording density, the gap between the two magnetic films must not exceed 20 nm.
Also, the crystal grains of the MgO film become larger the more the film thickness increases. The magnetic crystal grains which grow on these crystal grains are affected by the MgO crystal grain diameter. Hence the diameter of the crystal grains forming the magnetic film, which is the medium on which magnetic recording is performed at high density, must not exceed 20 nm but preferably does not exceed 15 nm, and it is preferable that the thickness of the MgO film is less than 13 nm.
When the soft magnetic backlayer has a polycrystalline structure, and even when it has an amorphous structure, it is effective to introduce a super-thin non-magnetic layer between the soft magnetic backlayer and MgO film to improve the (100) orientation characteristics of the MgO polycrystalline film. For this purpose a non-magnetic layer having an amorphous structure is particularly desirable. Materials which exhibit this desirable effect in the region when the film thickness does not exceed 10 nm are Ti, Zr, Hf, Cr, Mo, Nb, V, W, Si, Ge, B, C or alloys having these elements as their main component, or oxides chosen from the group SiO2, Al2O3 and ZrO2. If the MgO film is formed via the film of this non-magnetic material, its (100) orientation characteristics are largely improved.
The perpendicular magnetic layer of Co alloy may be formed on the (100) oriented MgO polycrystalline film, but to further promote noise reduction of the magnetic recording medium, it is effective to provide a super-thin non-magnetic layer having a hexagonal close-packed structure of several nm or less on the MgO film. By interposing this non-magnetic layer, the magnetic separation between magnetic crystals of the Co alloy perpendicular magnetic layer is particularly enhanced in the initial growth region. As both of these have the same hexagonal close-packed structure, epitaxial growth occurs wherein the crystal lattice grows continuously. This epitaxial growth is also effective in reducing crystal defects in the magnetic film and in achieving a desirable coercivity.
Examples of materials having a hexagonal close-packed structure with this effect are Coxe2x80x94Cr, Coxe2x80x94Crxe2x80x94X (Xxe2x95x90Mn, V, Zr, Hf, Nb, Mo, W, Si, B, Ta, Cu) where the addition amount of the non-magnetic element to Co exceeds 30 at %, or Ru, Ruxe2x80x94Y alloy (Yxe2x95x90Mn, Cr, Al, Cu), Ti, or Ti-Z alloy (Zxe2x95x90Co, Ni, Mn, Cu, Al).
In any case, for the magnetic recording medium to permit a recording density of 30 Gb/in2, the distance between the soft magnetic backlayer and the perpendicular magnetic layer comprising Co must not exceed 20 nm. The thickness of the MgO film in this case is in the range 1 nm to 15 nm, but more preferably less than 13 nm. Further, the perpendicular magnetic layer comprising Co is formed on the (100) oriented polycrystalline MgO film directly or via a non-magnetic layer having a hexagonal close packed structure, but perpendicular magnetic layers having other crystalline structures may also be formed on the perpendicular magnetic layer if necessary.